High quality remnant patches in a complex agricultural landscape sustain high spider diversity

Abstract

Remnant vegetation in agricultural landscapes can play a significant role in supporting farmland biodiversity, especially agriculturally important arthropods such as spiders. Little is known about spider composition in native vegetation remnant patches, or which environmental parameters influence within-patch diversity. Furthermore, data on spider diversity is limited in the Greater Cape Floristic Region (GCFR), a global botanical hotspot of great conservation value. Here we sampled spiders in 12 remnant natural fynbos (sclerophyllous) patches in a fragmented agricultural landscape. Multiple environmental variables were measured at each site (within patch variables and landscape variables), to determine their influence on spider genus richness and assemblage structure. We also did this for ground- and plant-dwelling genera, as well as for their assemblages separately. Overall, natural vegetation remnants supported high spider diversity, even rare and range restricted species. Both patch- and landscape-scale variables influenced spider diversity and assemblage structure. Remnant patch size influenced the assemblage structure of all guilds, and positively influenced overall spider diversity. Similarly, the amount of remnant vegetation in the landscape influenced the assemblage structure of most guilds, as well as positively influencing overall spider diversity, across multiple spatial scales. Additional important variables were rockiness, alien invasive tree species richness and topographic complexity. We show that spider diversity is maintained by remnants of good quality natural vegetation, but also influenced by the complexity of the landscape. Preservation of remnant vegetation patches, regardless of size, throughout the privately-owned farm conservancy landscape maintains spider diversity, and so private land conservation should be promoted.